1. Field of the Invention
The present invention relates to a metal working tool such as cutting tools and dies, made of a zirconia-type sintered composition, containing zirconia (ZrO.sub.2), alumina (Al.sub.2 O.sub.3) and/or spinel (MgAl.sub.2 O.sub.4) as well as a method for producing the sintered tool.
2. Description of the Prior Art
In the field of dies, conventionally dies, such as dies to be used in the hot and cold extrusion of metals and for the drawing and gathering of metal wires (hereinafter will simply be called die) have mostly been made of such metals as tool steels and super-hard alloys. However, these metal dies have an affinity to the metals to be extruded or drawn therethrough, and the metals easily stick to the dies, so that the end product will very often suffer from surface defects.
Recently, in the field of cutting tools, some types of ceramics have been found with increasing interest to be usable as high-speed cutting tools, and tools made of ceramics such as Al.sub.2 O.sub.3, Al.sub.2 O.sub.3 -TiC, Al.sub.2 O.sub.3 -TiN and thermets have actually been developed. However, while these tools demonstrate an excellent cutting property against such materials as iron and steel, they have a drawback such that the toughness is poor and they are easily broken. Further, since they have an affinity to such materials as aluminium alloys and copper alloys, they are not suitable for cutting them.
Under such circumstances, partially stabilized zirconia has been developed as a material to overcome the drawbacks of the conventional materials in the field of cutting tools and dies.
A sintered tetragonal zirconia with the addition of a small amount of Y.sub.2 O.sub.3 as a stabilizing agent (hereinafter called sintered Y-PSZ) has been proposed as material for a cutting tool for the cutting of so called "hard-to-cut" metals such as aluminum alloys (Japanese Patent Publication No. Sho 58-9784). However, these materials are not yet satisfactory in terms of the strength and hardness of the material.
A sintered material comprising a eutectic mixture of alumina and monoclinic and tetragonal zirconia produced by rapidly cooling the melt of binary system of zirconia and alumina has been developed for grinding stone. However, when the temperature in service of this material rises higher than 800.degree. C., and when the material is cooled from such a high temperature down to a temperature below 800.degree. C., the tetragonal crystals in the sintered material are transformed into monoclinic, causing the formation of cracks and collapse of the sintered structure. Therefore, this sintered material of eutectic structure of Al.sub.2 O.sub.3 and ZrO.sub.2 has a vital drawback that the grain-boundary strength is low. Application of this material to cutting tools and dies is dangerous when the working temperature exceeds 800.degree. C.
In normal high-speed cutting of metals, the tools are often exposed to a temperature as high as 1000.degree. C. In the field of dies, one made of a sintered zirconia material containing not less than 50 mol percent of tetragonal zirconia is disclosed in Japanese Patent Laid-Open Application No. Sho 58-161973. However, practical tests of this die have revealed that the die cracks during the test service, indicating that its mechanical strength is not satisfactory for the purposes.